twilight

twilight, period between sunset and total darkness or between total darkness and sunrise. Total darkness does not occur immediately when the sun sinks below the horizon because light from the sun that strikes the atmosphere is scattered (both by the air itself and by suspended matter, e.g., dust and smoke). Civil twilight ends when the center of the sun is 6° below the horizon. Although it is still not very dark, it is necessary to use artificial light to carry out most activities. Nautical twilight ends when the sun's center is 12° below the horizon; at about this time the light is too dim for the user of a sextant to see a sharp horizon. Astronomical twilight ends when the sun's center is 18° below the horizon; by this time even the faintest stars overhead can be seen. (Similar definitions apply to morning twilight.) During twilight, Venus or Mercury is often seen as the evening star or morning star. The length of twilight depends on latitude and the time of year. Twilight is generally shorter at the equator, where the sun's path toward the horizon is more nearly vertical than at higher latitudes; typically, astronomical twilight may last for 1 hr at the equator and 11/2 hr in New York City.